This invention relates to primary and secondary batteries for use downhole for powering a telemetry network and other downhole tools. More specifically, this invention relates to a battery or a fuel cell having internal components that are pressurized to the ambient pressure found downhole, to depths of 20,000 feet or more, in order to compensate for the downhole thermal conditions above 120° C.
Pressure and temperature conditions found downhole to depths of 20,000 feet or more are generally in excess of 10,000 psi and 200° C., respectively. Presently, the applicants know of no batteries commercially available that will function with high power and long life under such conditions.
The factors affecting downhole pressure are generally the depth of drilling, the hardness and continuity of the subterranean formations being drilled, the composition of the drilling fluid being used, and the under-balanced condition maintained in the well bore. Similar factors are also relevant to the temperature in the well bore. The extreme pressure and thermal conditions in deep wells contribute to premature battery degradation in downhole applications.
Generally, downhole batteries are self-contained in their own sealed containers that protect them from ambient conditions, including high pressure and temperature. The need to protect the battery components from external pressure requires an expensive pressure housing in the downhole tool, and the physical constraints imposed by the geometry of a downhole tool often mean that the batteries must intrude on the flow space within the tool, or they must be limited to relatively small diameters, reducing overall power density. Moreover, high pressure housings are always subject to leakage which may lead to a catastrophic failure of the battery and damage to the surrounding electronic components as well.
The researcher is referred to U.S. Pat. No. 6,187,469, to Marincie et al., incorporated herein, as an example of a downhole battery system. '469 teaches individual cells mounted end to end and interlocked together to prevent rotation of the cells relative to one another. The cells are electrically connected and mounted between an inner and outer tube. The housing for the battery is sufficiently strong to resist downhole pressures and is provided with vents for the discharge of gases.
Another reference of interest is U.S. Pat. No. 6,224,997, to Papadopoulos, incorporated herein. Papadopoulos teaches a flexible battery pack for powering downhole electronic equipment. The pack includes a plurality of electrochemical cells connected and tied together in a stacked relationship by a pair semi cylindrical encapsulating shells which form a primary containment for the cells. The shells comprise a glass fiber reinforced plastic material. The primary containment is sealed within a stainless steel tube which is provided to hold the components in place during operation. Electrical conductors are also provided to transmit the energy of the battery to selected downhole equipment. Each cell is sealed to resist the downhole environment and a venting mechanism is provided.
A pressure-equalized electrochemical battery system is disclosed in U.S. Pat. No. 4,087,590, to Kraft. Kraft discloses a pack of ordinary, commercially available batteries that are provided with a common electrolyte-filled reservoir coupled to each battery by a small tube. The reservoir includes a compliant diaphragm which, when exposed to pressure, applies the pressure to the reservoir of electrolyte to force it into the battery. Forceful filling the cells equalizes their internal and external pressures. Kraft is intended for deep-sea applications and would not meet the constraints of a downhole tool string.
U.S. Pat. No. 6,117,583, to Nilsson, discloses a battery that is intentionally pressurized, but it is pressurized above ambient pressure. In lead-acid batteries, the PbSO4 that formed during discharge has a higher volume than the Pb and PbO2 from which it is formed. When it is discharged the shrinkage causes loss of contact between the active components and the current conducting lead frame. This is overcome by providing a strong housing that keeps the battery under high pressure.
U.S. Pat. No. 6,253,847, to Stephensen, discloses a permanent downhole power source for a producing well, using the steel casing coated with iron oxide as the cathode. Fe2O3 is reduced to FeO. A separate piece of Zinc, as an anode, is provided which corrodes and is consumed. A cement treated with additives to increase its conductivity serves as the electrolyte.
Although the temperatures in some wells may exceed 200° C., the drilling fluid to drill those holes never boils, because the applied pressure exceeds the vapor pressure of the mud. Commercial batteries are generally limited to temperature conditions below 120° C. because of electrolyte boil off. A unique condition in a well is that there is a corresponding increase in pressure and temperature as the bore hole deepens. If it were possible to simultaneously expose the battery components subject to boil off to the increasing temperature and pressure, boil off would not likely occur, and the range of the battery's performance would be extended.
What is needed is a long life battery that will accommodate the physical constraints of downhole tools and be functional to depths to 20,000 feet or more.